This invention relates to electric power systems and, more particularly, to such systems having parallel-connected power supplies and methods of operating those supplies.
Often in fault-tolerant electric power systems, several (N) power supplies are connected in parallel for redundancy. Any one supply is capable of providing the entire load power. In DC systems, the supplies are usually connected to a common bus through diodes. This ensures isolation so that faulty supplies do not provide short circuit paths for active supplies. During normal operation, the supply with the slightly higher voltage becomes the active supply, providing all or most of the load current, while the others remain in an idle or standby mode. If the active supply should fail or its power source be cut off, the remaining supply with the highest voltage will become the new active supply.
While the above-described system provides redundancy, isolation, and independence in that there is no control interconnection, its efficiency is no better than that for a single supply system and possibly worse, since the idle supplies may still draw control power. The reliability of the overall system is high because of redundancy, but the reliability of the active supply is the same as that for a single supply since it is exposed to the full load voltage, current, and thermal stresses.
A better way to operate this type of system is to force load current sharing among the supplies. This increases efficiency since the resistive power losses (I.sup.2 R) are approximately 1/N of the losses of a single supply. Furthermore, power semiconductor device voltage drops, which contribute to power losses, are lower at 1/N of the full load current. Since each supply is exposed to less current and thermal stress, its reliability is higher than when carrying the entire load. This increases the overall system reliability above that of a system without load sharing, since overall system reliability is a function of the individual supply reliability. Another advantage is that heat removal may be easier since the losses are spread among the individual supplies.
Methods of accomplishing this improved system operation generally involve interconnecting the individual supply control circuits in a master-slave arrangement, or using a control circuit that automatically measures the current differences and adjusts the individual supplies to minimize them. In all cases, the lack of isolation and independence may make the system vulnerable to failure of an individual supply, current-sharing control circuit, or communication link, thereby defeating the purpose of redundancy. This may be especially true in applications that require supplies to be at some distance from each other, such as on aircraft or ships.
It is known that an increase in the output impedance of power supplies connected in parallel promotes current-sharing. This introduction of voltage regulation or droop makes the supply look slightly like a current source. With increasing droop, the supplies have more of a current source characteristic and current-sharing is more accurate. In the case of DC supplies, this impedance must be resistive. This, of course, is disadvantageous, resulting in increased losses and a fixed output droop characteristic at all load conditions.
It is therefore desirable to provide a power system comprised of N individual power supplies, parallel-connected to a common bus for redundancy, that can be operated in a current-sharing mode, gaining the advantages described above, without interconnection or communication among the supply control circuits, thereby gaining the additional advantages of total isolation and independence.